1. Field of the Invention
The present invention relates to a reproducing apparatus of a digital signal recorded on a recording medium such as an optical disk or the like.
2. Description of the Related Background Art
A Viterbi decoding method (Viterbi Algorithm) has been known as a method of decoding a digital signal with a high reliability from a read signal read out from a recording medium on which the digital signal has been recorded at a high density. In the Viterbi algorithm, a train of sample values which are obtained by sampling the read signal is regarded as a time sequence and a digital signal sequence of "1" and/or "0" which seems to be most certain is obtained on the basis of the time sequence.
FIG. 1 is a diagram showing a construction of a reproducing apparatus for reproducing a digital signal from an optical disk as an optical recording medium by applying the Viterbi algorithm.
In FIG. 1, an optical pickup 1 irradiates a light beam onto an optical disk 3 which is rotated by a spindle motor 2. A digital recording signal consisting of binary values each of which is "0" or "1" has been recorded on the optical disk 3. The optical pickup 1 photoelectrically converts a reflection light from the optical disk 3 and obtains a read signal p and supplies the signal to an A/D converter 10. The A/D converter 10 samples the read signal p at a predetermined sampling timing and obtains a sample value q and supplies it to a Viterbi decoder 20.
Explanation will now be made with respect to the sample value q which is obtained in the case where a reproducing system shown in FIG. 1 is a partial response system of PR (1, 2, 2, 1) and a recording signal recorded on the optical disk 3 is a signal that was modulated under the rule of RLL(1, 7).
In the reproducing system of PR (1, 2, 2, 1), a value to be obtained as a sample value q is decided every signal train of continuous four bits of the recording signal recorded on the optical disk 3. Since the recording signal has been modulated under the rule of RLL(1, 7), its minimum inversion interval is equal to 2.
When the recording signal recorded on the optical disk 3 is considered on a unit basis of the 4-bit signal train, therefore, there are only ten kinds of patterns of the 4-bit train as follows.
[0, 0, 0, 0] PA0 [0, 0, 0, 1] PA0 [0, 0, 1, 1] PA0 [0, 1, 1, 1] PA0 [1, 1, 1, 1] PA0 [1, 1, 1, 0] PA0 [1, 1, 0, 0] PA0 [1, 0, 0, 0] PA0 [0, 1, 1, 0] PA0 [1, 0, 0, 1]
When the PR (1, 2, 2, 1) transmitting system is considered by making bit "1" and bit "0" in the 4-bit signal train to correspond to +1 and -1, respectively, values of the sample values q which are obtained each time the 4-bit signal train is read out from the optical disk 3 become as follows. EQU q[0,0,0,0]=(-1).times.1+(-1).times.2+(-1).times.2+(-1).times.1=-6 EQU q[0,0,0,1]=(-1).times.1+(-1).times.2+(-1).times.2+(+1).times.1=-4 EQU q[0,0,1,1]=(-1).times.1+(-1).times.2+(+1).times.2+(+1).times.1=0 EQU q[0,1,1,1]=(-1).times.1+(+1).times.2+(+1).times.2+(+1).times.1=4 EQU q[1,1,1,1]=(+1).times.1+(+1).times.2+(+1).times.2+(+1).times.1=6 EQU q[1,1,1,0]=(+1).times.1+(+1).times.2+(+1).times.2+(-1).times.1=4 EQU q[1,1,0,0]=(+1).times.1+(+1).times.2+(-1).times.2+(-1).times.1=0 EQU q[1,0,0,0]=(+1).times.1+(-1).times.2+(-1).times.2+(-1).times.1=-4 EQU q[0,1,1,0]=(-1).times.1+(+1).times.2+(+1).times.2+(-1).times.1=2 EQU q[1,0,0,0]=(+1).times.1+(-1).times.2+(-1).times.2+(+1).times.1=-2
Namely, in the case where the reproducing system shown in FIG. 1 is the PR (1, 2, 2, 1) transmitting system and the recording signal recorded on the optical disk 3 has been modulated under the format of RLL(1, 7), a value to be predicted as a sample value q is set to any one of 6, 4, 2, 0, -2, -4, and -6.
A branch-metric calculation circuit 21 in the Viterbi decoder 20 obtains a square error, that is,
{[sample value q]-[prediction sample K]}.sup.2 PA1 prediction sample K.sub.0 =6 PA1 prediction sample K.sub.1 =4 PA1 prediction sample K.sub.2 =2 PA1 prediction sample K.sub.3 =0 PA1 prediction sample K.sub.4 =-2 PA1 prediction sample K.sub.5 =-4 PA1 prediction sample K.sub.6 =-6
between each of a plurality of prediction samples which can be predicted as a sample value q, namely,
and the actual sample value q, respectively, and supplies those square errors as branch-metric values to a path-metric calculation circuit 22.
FIG. 2 is a diagram showing an example of an internal construction of the branch-metric calculation circuit 21 for arithmetically calculating the branch-metric values by using the prediction samples K.sub.0 to K.sub.6.
In FIG. 2, the prediction samples K.sub.0 to K.sub.6 are fixedly supplied to subtracters 210 to 216, respectively. The subtracter 210 and a multiplier 217 calculate a square error between the sample value q supplied from the A/D converter 10 and the prediction sample K.sub.0 and supply the resulted square error as a branch-metric value e.sub.0 to the path-metric calculation circuit 22. The subtracter 211 and a multiplier 218 obtain a square error between the sample value q and the prediction sample K.sub.1 and supply it as a branch-metric value e.sub.1 to the path-metric calculation circuit 22. The subtracter 212 and a multiplier 219 obtain a square error between the sample value q and the prediction sample K.sub.2 and supply it as a branch-metric value e.sub.2 to the path-metric calculation circuit 22. The subtracter 213 and a multiplier 220 obtain a square error between the sample value q and the prediction sample K.sub.3 and supply it as a branch-metric value e.sub.3 to the path-metric calculation circuit 22. The subtracter 214 and a multiplier 221 obtain a square error between the sample value q and the prediction sample K.sub.4 and supply it as a branch-metric value e.sub.4 to the path-metric calculation circuit 22. The subtracter 215 and a multiplier 222 obtain a square error between the sample value q and the prediction sample K.sub.5 and supply it as a branch-metric value e.sub.5 to the path-metric calculation circuit 22. The subtracter 216 and a multiplier 223 obtain a square error between the sample value q and the prediction sample K.sub.6 and supply it as a branch-metric value e.sub.6 to the path-metric calculation circuit 22.
FIG. 3 is a diagram showing an example of the so-called eye pattern of the read signal p which is ideally obtained in the case where the reproducing system shown in FIG. 1 is the PR (1, 2, 2, 1) transmitting system and the recording signal recorded on the optical disk 3 is the RLL (1, 7) modulated signal.
The value of the sample value q which is obtained on the basis of the read signal p is equal to any one of the prediction samples K.sub.0 to K.sub.6. Any one of the branch-metric values e.sub.0 to e.sub.6 is, therefore, equal to 0. For example, when a sample value q.sub.0 is obtained at a sampling timing S2 shown in FIG. 3, the sample value q.sub.0 is equal to the prediction sample K.sub.0. In this instance, among the branch-metric values e.sub.0 to e.sub.6, the branch-metric value e.sub.0 is equal to 0. In the case where a sample value q.sub.6 is obtained at the sampling timing S.sub.2 shown in FIG. 3, the sample value q.sub.6 is equal to the prediction sample K.sub.6. In this instance, among the branch-metric values e.sub.0 to e.sub.6, the branch-metric value e.sub.6 is equal to 0.
The path-metric calculation circuit 22 individually obtains an accumulated addition value of each of the branch-metric values e.sub.0 to e.sub.6 every path and supplies a path selection signal indicative of a path in which the accumulated addition value is minimum to a path memory 23. While updating a serial digital signal sequence consisting of binary values of "0" and "1" in accordance with the path selection signal, the path memory 23 sequentially generates the updated digital signal sequence as a reproduction digital signal corresponding to the recording signal.
As mentioned above, the Viterbi decoder 20 obtains the square error values between the sample value q that is supplied from the A/D converter 10 and the prediction samples K.sub.0 to K.sub.6, respectively, and generates the data sequence corresponding to the path in which the accumulated addition value of the square error values is minimum as a reproduction digital signal corresponding to the recording signal.
When an asymmetry occurs in the read signal p and its signal waveform becomes asymmetrical with respect to the center level, however, the sample value q is not equal to any one of the prediction samples K.sub.0 to K.sub.6, so that such a problem occurs that a decoding performance of the Viterbi decoder deteriorates.
FIG. 4 is a diagram showing an example of the eye pattern in case an asymmetry occurs in the read signal p.
In FIG. 4, when the sample value obtained at the sampling timing S.sub.2 is equal to q.sub.0, the sample value q.sub.0 is not equal to any one of the prediction samples K.sub.0 to K.sub.6. In this instance, an error .DELTA.q occurs even for the prediction sample K.sub.0 existing at the nearest position. Since the error .DELTA.q occurs, each of the branch-metric values e.sub.0 to e.sub.6 increases. In the Viterbi decoding for obtaining a most certain data signal sequence on the basis of the accumulated addition value of the branch-metric value, consequently, its decoding performance deteriorates.